The respiratory tract is constantly exposed to microbial pathogens and particles, but is protected by a multitiered host defense system that serves to maintain lung function and sterility. Severe defects in mucociliary clearance, mucus production, and host defense accompany common chronic lung diseases, including cystic fibrosis (CF), chronic obstructive pulmonary disease, and asthma. These disorders are complicated by mucus metaplasia, mucus hyperproduction or inspissation, inflammation, and susceptibility to pulmonary infection. This application seeks to determine the molecular mechanisms underlying deficits in mucociliary clearance associated with pulmonary disease in CF. The work is based on preliminary data demonstrating 1) a novel network of transcription factors that determines both the patterning and differentiation of airway epithelial cells (AECs) lining the developing trachea and bronchi, and the formation of submucosal glands (SMGs) that secrete the majority of fluids, electrolytes, and host defense proteins onto the airway surface and 2) that patterning, growth, diferentiation, and gene expression of AECs and SMGs are influenced by the lack of Cystic Fibrosis Transmembrane Conductance Regulator (CFTR). This application will test the hypothesis that PAX9 and an associated transcriptional network play a critical role in the regulation of AEC and SMG morphogenesis and function in the developing and mature airway. This proposal will utilize transgenic mice in which PAX9 and genes associated with PAX9 are conditionally deleted or added to the developing and mature airway epithelium in vivo and in vitro. The molecular and cellular mechanisms by which PAX9 and associated proteins regulate genes and processes critical for AEC and SMG formation and function wil be assessed. The role of the proposed PAX9-dependent regulatory program in the pathogenesis of the pulmonary complications of CF will be determined in CFTR-deficient pigs and mice. This proposal seeks to determine the cellular and molecular basis underlying AEC and SMG morphogenesis and function relevant to the pathogenesis of recurrent infections caused by defects in mucociliary clearance.